A typical lithium-sulphur cell comprises an anode (negative electrode) formed from lithium metal or a lithium metal alloy, and a cathode (positive electrode) formed from elemental sulphur or other electroactive sulphur material. The sulphur or other electroactive sulphur-containing material may be mixed with an electrically conductive material, such as carbon, to improve its electrical conductivity. Typically, the carbon and sulphur are ground and then mixed with solvent and binder to form a slurry. The slurry is applied to a current collector and then dried to remove the solvent. The resulting structure is calendared to form a composite structure, which is cut into the desired shape to form a cathode. A separator is placed on the cathode and a lithium anode placed on the separator. Electrolyte is then introduced into the assembled cell to wet the cathode and separator.
Lithium-sulphur cells are secondary cells and may be recharged by applying an external current to the cell. Typically, the cell is charged to a fixed cut-off voltage of, for example, 2.45-2.8V. However, with repeated cycling over an extended period, the capacity of the cell may fade. Accordingly, by repeatedly charging the cell to the selected cut-off voltage, the cell may eventually be repeatedly over-charged. This can have a detrimental effect on the longevity of the cell, as undesirable chemical reactions may take lead to damage to, for example, the cell's electrodes and/or electrolytes.
A method for terminating the charging of a lithium-sulphur cell is described in WO 2007/111988. Specifically, this reference describes adding an N—O additive, such as lithium nitrate, to the electrolyte of the cell. According to the passage at page 16, lines 29 to 31, of this reference, the additive is effective in providing a charge profile with a sharp increase in voltage at the point of full charge. Accordingly, if the cell voltage during charge is monitored, charging can be terminated once a rapid increase in voltage is observed.
The method of WO 2007/111988 relies on the voltage of the cell increasing sharply as the cell reaches full capacity. Not all lithium-sulphur cells, however, exhibit such a charging profile.